The proposed study is intended to investigate the possibility of utilizing the naturally occurring cellular antioxidant mechanisms to increase tolerance to the toxic effects of oxygen at increased partial pressures above 1ATA (one atmosphere absolute or sea level). This will be approached by examining the ability of various patterns of exposure as well as certain exogenously administered agents to increase the tolerance of rats exposed to oxygen at increased pressures; attempting to relate this altered tolerance to an enhancement of cellular antioxidant mechanisms. One such pattern of exposure which has shown to result in increased tolerance (as evidenced by increased time to onset of seizures) of rats exposed to 3ATA oxygen is the pretreatment of these rats with exposure to 4ATA oxygen for one hour immediately prior to the 3ATA exposure. This will be used as the starting point for the proposed experiments. The exposures and pretreatments will be designed to stimulate the existing endogenous cellular antioxidant mechanisms, providing increased levels of endogenous substances and enzymes (such as superoxide dismutase, glutathione, glutathione reductase and glucose-6-phosphate dehydrogenase) which should then serve to increase the tolerance of the animals subsequently exposed to oxygen at high pressure.